1. Field of the Invention
This invention relates to a simultaneous fermentation/separation system (as well as apparatus and methods for use thereof) particularly useful in carrying out product inhibited fermentations such as in the bio-production of ethanol, acetone, acetic acid, butanediol, butanol or lactic acid. More particularly, this invention relates to a novel continuous cell reactor-separator which incorporates a unique four phase system design (e.g., (1) an inert stripping gas phase, (2) the liquid fermentation broth, (3) the solid reactor internals, and (4) the biological catalyst or cells), wherein any inhibitory metabolite is separated from the fermentation medium as soon as formed. In its preferred embodiments the continuous cell reactor-separator of this invention combines a co-current gas-liquid flow reactor system with a counter-current gas-liquid flow reactor system into a reactor design which produces a final effluent stream with both very low product and substrate concentrations, and a purified product stream containing no fermentation substrate or cells. In essence the preferred continuous cell reactor-separator system of this invention combine the advantages of a conventional plug reactor etc. reducing the liquid product concentration so that high reaction rates are maintained.
2. Description of the Problem and Prior Art
The production of inhibitory products or by-products during the course of a fermentation causes growth rates and reaction reaction rates to slow and eventually results in cell death. If the inhibitory compounds are removed as they are formed, then the reaction rates and cell viability can be maintained. Ethanol fermentation is a product inhibited reaction with a maximum final tolerable ethanol concentration ranging from 6 to 15% (W/V) depending upon the micro-organism. If the ethanol can be removed during the fermentation then high reaction rates and good substrate utilization can be assured. There have been several attempts to achieve this simultaneous separation-fermentation. Ramalingham and Finn (1977) and Cysewski and Wilke (1977) ran a continuous stirred tubular type reactor (CSTR) under vacuum. Spear (1981) suggested continuously withdrawing a portion of a fermenting mixture from a CSTR, flashing this stream to an ethanol enriched vapor output and ethanol depleted liquid which may then be returned to the reactor. Minier and Goma (1982) recently suggested a liquid extraction of ethanol using dodecanol in a pulsed cocurrent immobilized cell tubular reactor. They found rates increased by a factor of four and high sugar levels (400 gm/l) were well utilized. Herein we describe a gas-liquid separation in an immobilized cell bioreactor.